Air Conditioning Clinic

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1 Air Conditioning Clinic Cooling and Heating Load Estimation One of the Fundamental Series April 2011 TRG-TRC002-EN

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3 Cooling and Heating Load Estimation One of the Fundamental Series A publication of Trane, a business of Ingersoll Rand

4 Preface Figure 1 The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience. The Trane Air Conditioning Clinic series is one means of knowledge sharing. It is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning. We have taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text. This particular clinic introduces the reader to cooling and heating load estimation. It is intended to introduce the concepts of estimating building cooling and heating loads and is limited to introducing the components that make up the load on a building, the variables that affect each of these components, and simple methods used to estimate these load components. It is not intended to teach all the details or latest computerized techniques of how to calculate these loads. If you are interested in learning more about the specific techniques used for cooling and heating load estimating, this booklet includes several references in the back. ii 2011 Trane. All rights reserved TRG-TRC002-EN

7 period one Human Comfort Cooling and Heating Load Estimation period one Human Comfort Figure 2 Heating and air conditioning systems use the principles of heat transfer to maintain comfortable indoor conditions for people. Principles of Heat Transfer Heat energy cannot be destroyed Heat always flows from a higher temperature substance to a lower temperature substance Heat can be transferred from one substance to another Figure 3 The three basic principles of heat transfer discussed in this clinic are: 1) Heat energy cannot be destroyed; it can only be transferred to another substance. To produce cooling, heat must be removed from a substance by transferring the heat to another substance. This is commonly referred to as the principle of conservation of energy. Ice cubes are typically placed in a beverage to cool it before being served. As heat is transferred from the beverage to the ice, the temperature of the beverage is lowered. The heat removed from the beverage is not destroyed, but instead is absorbed by the ice, melting the ice from a solid to a liquid. TRG-TRC002-EN 1

8 period one Human Comfort 2) Heat energy naturally flows from a higher-temperature substance to a lowertemperature substance, in other words, from hot to cold. Heat cannot naturally flow from a cold substance to a hot substance. Consider the example of the beverage and the ice cubes. Because the temperature of the beverage is higher than the temperature of the ice cubes, heat will always flow from the beverage to the ice cubes. 3) Heat energy is transferred from one substance to another by one of three basic processes: conduction, convection, or radiation. Methods of Heat Transfer convection warm air radiation hot water conduction cool air Figure 4 The device shown is a baseboard convector that is commonly used for heating a space. It can be used to demonstrate all three processes of transferring heat. Hot water flows through a tube inside the convector, warming the inside surface of the tube. Heat is transferred, by conduction, through the tube wall to the slightly cooler fins that are attached to the outside surface of the tube. Conduction is the process of transferring heat through a solid. The heat is then transferred to the cool air that comes into contact with the fins. As the air is warmed and becomes less dense, it rises, carrying the heat away from the fins and out of the convector. This air movement is known as a convection current. Convection is the process of transferring heat as the result of the movement of a fluid. Convection often occurs as the result of the natural movement of air caused by temperature (density) differences. Additionally, heat is radiated from the warm cabinet of the convector and warms cooler objects within the space. Radiation is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects. An interesting thing about radiated heat is that it does not heat the air between the source and the object it contacts; it only heats the object itself. 2 TRG-TRC002-EN

9 period one Human Comfort Measuring Heat Quantity 1 lb water 1 Btu 60 F 61 F 1 kg water 1 kcal 15 C [4.19 kj] 16 C Figure 5 In the I-P system of units, the unit for measuring the quantity of heat is the British Thermal Unit (Btu). The Btu is defined as the quantity of heat energy required to raise the temperature of 1 lb of water 1 F. Similarly, in the Système International (SI) system, heat quantity can be expressed using the unit kilojoule (kj). A kcal is defined as the amount of heat energy required to raise the temperature of 1 kg of water 1 C. One kcal is equal to 4.19 kj. In heating and cooling applications, however, emphasis is placed on the rate of heat transfer, that is, the quantity of heat that flows from one substance to another within a given period of time. This rate of heat flow is commonly expressed in terms of Btu/hr the quantity of heat, in Btu, that flows from one substance to another during a period of 1 hour. Similarly, in the SI system of units, the rate of heat flow is expressed in terms of kilowatts (kw). One kw is equivalent to 1 kj/sec. One kilowatt describes the quantity of heat, in kj, that flows from one substance to another during a period of 1 second. Finally, the rate of heat flow may often be expressed in terms of watts (W). One kw is equivalent to 1000 W. TRG-TRC002-EN 3

10 period one Human Comfort Sensible versus Latent Heat sensible heat 60 F [15.6 C] 212 F [100 C] 212 F [100 C] latent heat 212 F [100 C] Figure 6 The process of comfort heating and air conditioning is simply a transfer of energy from one substance to another. This energy can be classified as either sensible or latent heat energy. Sensible heat is heat energy that, when added to or removed from a substance, results in a measurable change in dry-bulb temperature. Changes in the latent heat content of a substance are associated with the addition or removal of moisture. Latent heat can also be defined as the hidden heat energy that is absorbed or released when the phase of a substance is changed. For example, when water is converted to steam, or when steam is converted to water. Heat Generated by People Figure 7 The necessity for comfort air conditioning stems from the fact that the metabolism of the human body normally generates more heat than it needs. This heat is transferred by convection and radiation to the environment 4 TRG-TRC002-EN

11 period one Human Comfort surrounding the body. The average adult, seated and working, generates excess heat at the rate of approximately 450 Btu/hr [132 W]. About 60% of this heat is transferred to the surrounding environment by convection and radiation, and 40% is released by perspiration and respiration. As the level of physical activity increases, the body generates more heat in proportion to the energy expended. When engaged in heavy labor, as in a factory for example, the body generates 1,450 Btu/hr [425 W]. At this level of activity, the proportions reverse and about 40% of this heat is transferred by convection and radiation and 60% is released by perspiration and respiration. Surrounding Air Conditions Figure 8 In order for the body to feel comfortable, the surrounding environment must be of suitable temperature and humidity to transfer this excess heat. If the temperature of the air surrounding the body is too high, the body feels uncomfortably warm. The body responds by increasing the rate of perspiration in order to increase the heat loss through evaporation of body moisture. Additionally, if the surrounding air is too humid, the air is nearly saturated and it is more difficult to evaporate body moisture. If the temperature of the air surrounding the body is too low, however, the body loses more heat than it can produce. The body responds by constricting the blood vessels of the skin to reduce heat loss. TRG-TRC002-EN 5

12 period one Human Comfort Factors Affecting Human Comfort Dry-bulb temperature Humidity Air movement Fresh air Clean air Noise level Adequate lighting Proper furniture and work surfaces Figure 9 The term comfort is often used to define a broader set of conditions than just temperature and humidity. Air movement, adequate fresh air, cleanliness of the air, noise levels in the space, adequate lighting, and proper furniture and work surfaces, are just a few of the other variables that contribute to making a space comfortable for its occupants. This clinic, however, will focus only on the aspects of thermal comfort. Thermal comfort depends on creating an environment of dry-bulb temperature, humidity, and air motion, that is appropriate for the activity level of the people in the space. This environment allows the body s rate of heat generation to balance with the body s rate of heat loss. 6 TRG-TRC002-EN

13 period one Human Comfort Indoor Design Conditions wet-bulb wet bulb temperature temperature A % RH RH RH comfort zone 70 F 80 F [21.2 C] [26.7 C] dry-bulb temperature 30 % RH humidity ratio Figure 10 Research studies have been conducted to show that, with a specific amount of air movement, thermal comfort can be produced with certain combinations of dry-bulb temperature and relative humidity. When plotted on a psychrometric chart, these combinations form a range of conditions for delivering acceptable thermal comfort to 80% of the people in a space. This comfort zone and the associated assumptions are defined by ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy. Determining the desired condition of the space is the first step in estimating the cooling and heating loads for the space. In this clinic, we will choose 78ºF [25.6ºC] dry-bulb temperature and 50% relative humidity (A) as the desired indoor condition during the cooling season. TRG-TRC002-EN 7

14 period two Cooling Load Estimation Cooling and Heating Load Estimation period two Cooling Load Estimation Figure 11 The selection of heating, ventilating, and air conditioning (HVAC) system components and equipment should always be based on an accurate determination of the building heating and cooling loads. During this period we will estimate the cooling loads for a single space in a single-story office building. In Period Four we will estimate the heating loads for this same space. As stated in the preface, this clinic is intended to introduce the concepts of estimating building cooling and heating loads and is not intended to cover all of the details. The Cooling Load Temperature Difference/Solar Cooling Load/ Cooling Load Factor (CLTD/SCL/CLF) load estimation method, used throughout Period Two, is a simplified hand calculation procedure developed long ago by ASHRAE. Because of its simplicity, it is the most common method used for basic instruction on estimating cooling loads. 8 TRG-TRC002-EN

15 period two Cooling Load Estimation Cooling Load Components roof glass solar glass conduction infiltration people lights equipment partition wall exterior wall floor Figure 12 The space cooling load is the rate at which heat must be removed from a space in order to maintain the desired conditions in the space, generally a dry-bulb temperature and relative humidity. The cooling load for a space can be made up of many components, including: Conduction heat gain from outdoors through the roof, exterior walls, skylights, and windows. (This includes the effects of the sun shining on these exterior surfaces.) Solar radiation heat gain through skylights and windows. Conduction heat gain from adjoining spaces through the ceiling, interior partition walls, and floor. Internal heat gains due to people, lights, appliances, and equipment in the space. Heat gain due to hot, humid air infiltrating into the space from outdoors through doors, windows, and small cracks in the building envelope. In addition, the cooling coil in the building HVAC system has to handle other components of the total building cooling load, including: Heat gain due to outdoor air deliberately brought into the building for ventilation purposes. Heat generated by the fans in the system and possibly other heat gains in the system. Throughout this period, we will assume that the space has no plenum (the space between the ceiling and roof). Therefore, all of the heat gain due to the roof and lighting affects the space directly. TRG-TRC002-EN 9

16 period two Cooling Load Estimation Cooling Load Components cooling load components sensible load conduction through roof, walls, windows, and skylights solar radiation through windows, skylights conduction through ceiling, interior partition walls, and floor people lights equipment/appliances infiltration ventilation system heat gains latent load space load coil load Figure 13 These load components contribute sensible and/or latent heat to the space. Conduction through the roof, exterior walls, windows, skylights, ceiling, interior walls, and floor, as well as the solar radiation through the windows and skylights, all contribute only sensible heat to the space. The people inside the space contribute both sensible and latent heat. Lighting contributes only sensible heat to the space, while equipment in the space may contribute only sensible heat (as is the case for a computer) or both sensible and latent heat (as is the case for a coffee maker). Infiltration generally contributes both sensible and latent heat to the space. The cooling coil has to handle the additional components of ventilation and system heat gains. Ventilation contributes both sensible and latent heat to the coil load. Other heat gains that occur in the HVAC system (from the fan, for example) generally contribute only sensible heat. 10 TRG-TRC002-EN

17 period two Cooling Load Estimation Time of Peak Cooling Load heat gain east-facing window roof mid a.m. noon p.m. mid Figure 14 One of the more difficult aspects of estimating the maximum cooling load for a space is determining the time at which this maximum load will occur. This is because the individual components that make up the space cooling load often peak at different times of the day, or even different months of the year. For example, the heat gain through the roof will be highest in the late afternoon, when it is warm outside and the sun has been shining on it all day. Conversely, the heat gain due to the sun shining through an east-facing window will be highest in the early morning when the sun is rising in the east and shining directly into the window. Determining the time that the maximum total space cooling load occurs will be discussed later in this clinic. Example Office Space (Room 101) North Room 101 Room 102 elevation view (Room 101) plan view Figure 15 Room 101 is the space that we will use as an example throughout this clinic. The windows face west and the solar heat gain through these windows will TRG-TRC002-EN 11

19 period two Cooling Load Estimation Outdoor Design Conditions 0.4% 1% 2% St. Louis, Missouri DB WB DB WB DB WB 95 F 76 F 93 F 75 F [35 C] [25 C] [34 C] [24 C] 90 F 74 F [32 C] [23 C] Figure 16 Outdoor Design Conditions In Period One, we discussed the indoor conditions required for thermal comfort. The next step toward estimating the cooling load of a space is to determine the highest, frequently-occurring outdoor air temperature. In the summer, for example, when the temperature outside is high, heat transfers from outdoors to indoors, thus contributing to the heat gain of the space. Obviously, HVAC systems would be greatly oversized if cooling load calculations were based on the most extreme outdoor temperature ever recorded for the location. Instead, outdoor design temperatures are based on their frequency of occurrence. Design outdoor conditions for many locations can be found in the ASHRAE Handbook Fundamentals. Figure 16, the cooling outdoor design conditions for St. Louis, Missouri, includes three columns of dry-bulb temperatures and corresponding wet-bulb temperatures. The first column heading, 0.4%, means that the dry-bulb temperature in St. Louis exceeds 95ºF [35ºC] for only 0.4% of all of the hours in an average year (or 35 hours). Also, 76ºF [25ºC] is the wet-bulb temperature that occurs most frequently when the dry-bulb temperature is 95ºF [35ºC]. The second column heading, 1%, means that the temperature exceeds 93ºF [34ºC] for only 1% of all of the hours in an average year (or 87.6 hours). When the drybulb temperature is 93ºF [34ºC], the wet-bulb temperature that occurs most frequently is 75ºF [24ºC]. For our example, we will use the more severe 95ºF [35ºC] dry bulb and 76ºF [25ºC] wet bulb for the outdoor design conditions. The tables published by ASHRAE include more weather data that can be useful for sizing certain HVAC system components, but that discussion is outside the scope of this clinic. TRG-TRC002-EN 13

20 period two Cooling Load Estimation Heat Conduction through Surfaces Figure 17 Conduction through Surfaces Conduction is the process of transferring heat through a solid, such as a wall, roof, floor, ceiling, window, or skylight. Heat naturally flows by conduction from a higher temperature to a lower temperature. Generally, when estimating the maximum cooling load for a space, the temperature of the air outdoors is higher than the temperature of the air indoors. We will focus on the most common conduction heat gains to a space: through the roof, external walls, and windows. Conduction through a Shaded Wall Q = U A ΔT Figure 18 Although often not applicable, a simplifying assumption when estimating the conduction heat gain through an exterior surface is to assume that the surface is completely shaded at all times. With this assumption, the amount of heat transferred through the surface is a direct result of the temperature difference between the space and outdoors. This assumption, however, does not include 14 TRG-TRC002-EN

21 period two Cooling Load Estimation the additional heat transfer that occurs because of the sun shining on the surface. This will be discussed next. The amount of heat transferred through a shaded exterior surface depends on the area of the surface, the overall heat transfer coefficient of the surface, and the dry-bulb temperature difference from one side of the surface to the other. The equation used to predict the heat gain by conduction is: Q = U A ΔT where, Q = heat gain by conduction, Btu/hr [W] U = overall heat-transfer coefficient of the surface, Btu/hr ft 2 F [W/m 2 K] A = area of the surface, ft 2 [m 2 ] ΔT = dry-bulb temperature difference across the surface, ºF [ C] In the case of a shaded exterior surface, this temperature difference is the design outdoor dry-bulb temperature (T o ) minus the desired indoor dry-bulb temperature (T i ). U-factor wood studs insulation concrete block aluminum siding gypsum board Figure 19 The overall heat transfer coefficient is also called the U-factor. The U-factor describes the rate at which heat will be transferred through the structure. Walls and roofs are typically made up of layers of several materials. The U-factor for a specific wall or roof is calculated by summing the thermal resistances (R-values) of each of these layers and then taking the inverse. The ASHRAE Handbook Fundamentals tabulates the thermal resistance of many common materials used in constructing walls, roofs, ceilings, and floors. TRG-TRC002-EN 15

24 period two Cooling Load Estimation Sunlit Surfaces sun rays solar angle changes throughout the day Figure 22 Most exterior surfaces of a building, however, are exposed to direct sunlight during some portion of the day. Solar heat energy is generated by the sun and radiated to earth. Radiant heat is similar to light, in that it travels in a straight line and can be reflected from a bright surface. Both light and radiant heat can pass through a transparent surface (such as glass), yet neither can pass directly through an opaque or non-transparent surface (such as a brick wall). When the sun s rays strike an opaque surface, however, a certain amount of radiant heat energy is transferred to that surface, resulting in an increase in the surface temperature. The amount of heat transferred depends primarily on the color and smoothness of the surface, and the angle at which the sun s rays strike the surface. When the sun s rays strike the surface at a 90º angle, the maximum amount of radiant heat energy is transferred to that surface. When the same rays strike that same surface at a lesser angle, less radiant heat energy is transferred to the surface. The angle at which the sun s rays strike a surface depends upon the latitude, the time of day, and the month of the year. Due to the rotation of the earth throughout the day, and the earth orbiting the sun throughout the year, the angle at which the sun s rays strike a surface of a building is constantly changing. This varies the intensity of the solar radiation on an exterior surface of a building, resulting in a varying amount of solar heat transferred to the surface throughout the day and throughout the year. As mentioned previously, the assumption that the surface is completely shaded does not account for the additional heat gain that occurs when the sun shines on a surface. Solar heat, therefore, must be considered, as it constitutes an important part of the total cooling load of most buildings. 18 TRG-TRC002-EN

25 period two Cooling Load Estimation Time Lag time lag solar effect A B mid a.m. noon p.m. mid Figure 23 The walls and roof that make up a building s envelope have the capacity to store heat energy. This property delays the heat transfer from outdoors to the space. The time required for heat to be transferred through a structure into the space is called the time lag. For example, the heat that is transferred through a sunlit wall into a space is the result of sunlight that fell on the outer surface of the wall earlier in the day. Curve A shows the magnitude of the solar effect on the exterior wall. Curve B shows the resulting heat that is transferred through the wall into the space. This delay in the heat gain to the space is the time lag. The magnitude of this time lag depends on the materials used to construct the particular wall or roof, and on their capacity to store heat. TRG-TRC002-EN 19

26 period two Cooling Load Estimation Conduction through Sunlit Surfaces Q = U A CLTD Figure 24 A factor called the cooling load temperature difference (CLTD) is used to account for the added heat transfer due to the sun shining on exterior walls, roofs, and windows, and the capacity of the wall and roof to store heat. The CLTD is substituted for ΔT in the equation to estimate heat transfer by conduction. Q = U A CLTD CLTD Factors for West-Facing Wall CLTD ( F) CLTD ( C) hour Figure 25 This particular table includes CLTD factors for a west-facing wall similar to the type used in our example building. It should be noted that the data in this table are based on the following assumptions: 78ºF [25.6ºC] indoor air 95ºF [35ºC] maximum outdoor air Average outdoor daily temperature range of 21ºF [11.7ºC] 20 TRG-TRC002-EN

27 period two Cooling Load Estimation 21 st day of July 40º north latitude Dark-colored surface Tables for various wall and roof types, as well as correction factors for applications that differ from these assumptions, can be found in the 1997 ASHRAE Handbook Fundamentals and ASHRAE s Cooling and Heating Load Calculation Principles manual. The wall in our example is classified as Wall Type 9. At 4 p.m. (Hour 17 in this table), the CLTD for a west-facing wall of this type is 22ºF [12 C]. This means that, even though the actual dry-bulb temperature difference is only 17ºF (95ºF 78ºF) [9.4ºC (35ºC 25.6ºC)], the sun shining on the outer surface of this wall increases the effective temperature difference to 22ºF [12 C]. Notice that the CLTD increases later in the day, and then begins to decrease in the evening as the stored heat is finally transferred from the wall into the space. Table 1. CLTDs for Sunlit Walls (40 North Latitude, July 21), F Wall Type 9 Hour N NE E SE S SW W NW Table 2. CLTDs for Sunlit Walls (40 North Latitude, July 21), C Wall Type 9 Hour N NE E SE S SW W NW Source: 1997 ASHRAE Handbook Fundamentals, Chapter 28, Table 32 TRG-TRC002-EN 21

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